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Re: [Qemu-devel] [PATCH] docs: clarify that qcow2 file size is not alway
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] [PATCH] docs: clarify that qcow2 file size is not always a cluster multiple |
Date: |
Wed, 28 May 2014 13:29:33 +0200 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
Am 27.05.2014 um 19:45 hat Benoît Canet geschrieben:
> The Tuesday 27 May 2014 à 17:29:16 (+0200), Paolo Bonzini wrote :
> > Il 27/05/2014 18:00, Benoît Canet ha scritto:
> > >> Are you worried that the host file system will lay out data poorly
> > >> because the file looks like this?
> > >>
> > >> | header (1C) | refcounts (2C) | L1 (512B) | hole | Next cluster |
> > >>
> > >> B = bytes
> > >> C = clusters
> > >>
> > >> My guess is the next cluster will be aligned to a reasonable boundary on
> > >> the physical disk.
> > >
> > >I have some kind of doubt. Does anyone knows a filesystem guru ?
> >
> > Not a guru, but indeed there is a risk that the layout will be worse
> > than necessary.
> >
> > I think holes are ignored unless they are big enough, but 64K-512 is
> > probably enough to create one. Indeed here I get this:
> >
> > $ qemu-img create -f qcow2 foo.qcow2 10G
> > $ qemu-io -c 'write 0 512' foo.qcow2
> > $ qemu-img map -f raw foo.qcow2 10G
> > Offset Length Mapped to File
> > 0 0x31000 0 foo.qcow2
> > 0x40000 0x20000 0x40000 foo.qcow2
>
> That seems to be 4k aligned. It's better than I though.
These 4k are probably the host file system block size.
Kevin
- [Qemu-devel] [PATCH] docs: clarify that qcow2 file size is not always a cluster multiple, Stefan Hajnoczi, 2014/05/22
- Re: [Qemu-devel] [PATCH] docs: clarify that qcow2 file size is not always a cluster multiple, Eric Blake, 2014/05/22
- Re: [Qemu-devel] [PATCH] docs: clarify that qcow2 file size is not always a cluster multiple, Kevin Wolf, 2014/05/23
- Re: [Qemu-devel] [PATCH] docs: clarify that qcow2 file size is not always a cluster multiple, Benoît Canet, 2014/05/26
- Re: [Qemu-devel] [PATCH] docs: clarify that qcow2 file size is not always a cluster multiple, Markus Armbruster, 2014/05/27
- Re: [Qemu-devel] [PATCH] docs: clarify that qcow2 file size is not always a cluster multiple, Stefan Hajnoczi, 2014/05/28